-
- News
- Books
Featured Books
- smt007 Magazine
Latest Issues
Current IssueBox Build
One trend is to add box build and final assembly to your product offering. In this issue, we explore the opportunities and risks of adding system assembly to your service portfolio.
IPC APEX EXPO 2024 Pre-show
This month’s issue devotes its pages to a comprehensive preview of the IPC APEX EXPO 2024 event. Whether your role is technical or business, if you're new-to-the-industry or seasoned veteran, you'll find value throughout this program.
Boost Your Sales
Every part of your business can be evaluated as a process, including your sales funnel. Optimizing your selling process requires a coordinated effort between marketing and sales. In this issue, industry experts in marketing and sales offer their best advice on how to boost your sales efforts.
- Articles
- Columns
Search Console
- Links
- Events
||| MENU - smt007 Magazine
Mastering Mixed Assembly Processing
December 31, 1969 |Estimated reading time: 10 minutes
BY Zulki Khan, NexLogic Technologies Inc.
Surface mount has been the prevalent assembly technology for the last 10 to 15 years. However, several OEM systems such as mil/aero and medical electronics continue holding on to the vestiges of thru-hole assembly, while adopting greater use of SMT assembly. SMT and thru-hole mixed-assembly placement will continue to be used for the foreseeable future. Design for manufacturing (DfM) is the linchpin and great equalizer for assuring accurate mixed-assembly placement.
Thru-hole has created its own niche over time and has remained in certain PCB applications because the I/O communications portions of a PCB design rely on thru-hole-based I/O ribbon cables and other connectors. More recently, SMT-based connectors have come on the market, which are now gaining popularity.
SMT and thru-hole mixed assembly placement will continue to be used in the foreseeable future for at least two reasons. One is OEMs’ reluctance to change. The other is that thru-hole currently is the accepted method for I/O communication going into and outside of the PCB. In these instances, mixed assembly placement requires special considerations and poses certain limitations and major implications.
Multiple reasons keep OEM products using thru-hole and thru-hole design and assembly in an EMS provider’s arsenal. An older, mature product is involved and there’s no valid reason for changing it. It’s certified and approved by outside agencies such as FDA, FAA, and UL. It’s highly successful in the marketplace; replacing its components is difficult; generally only thru-hole ones are available and as such the product can only be manufactured using thru-hole components.
Figure 1. Thru-hole components must be adequately separated from SMDs to allow sufficient “tie back” space.
Some mil/aero and medical electronics systems are prime examples of these older, highly accepted, time-tested products. Thru-hole is at the heart of these products, and they are difficult to change for several major reasons, the main one being the product has continued to successfully operate. In most cases, special SMT and thru-hole techniques have been successfully adopted, proven, and continually used over the years. A mil/aero PCB designed for rugged environments and populated with heavy and wide components is one such example. Thru-hole components must be adequately separated from SMT components to allow sufficient tie- or wrap-back space (Figure 1). This technique securely holds down thru-hole components to minimize physical movement and assure a component and its solder joints remain sturdy and firm.
Figure 2. RTV silicone sealant is applied between tall thru-hole components to restrict physical movement and maintain solder joint integrity.
A similar mil/aero and medical electronics PCB technique successfully applied over time involves separating SMDs and tall thru-hole components. This technique is applied when multiple tall thru-hole components are found close to each other. Probability of colliding, thus causing solder joints fractures, is high. The thru-hole components must therefore be placed a sufficient distance away from SMT components, allowing enough room for rework, if needed. Room-temperature vulcanizing (RTV) silicone sealant is applied between these tall thru-hole components, restricting their physical movement and maintaining solder joint integrity (Figure 2).
To change a mil/aero or medical electronics system like these would require lengthy approval cycles of two to three months, plus the sign-offs of an entire chain of command (mil/aero) and a lengthy list of agency approvals (medical). In addition, a large amount of paperwork and documentation is required, dramatically increasing product development costs. To avoid inordinate time and cost obstacles, high-reliability electronics OEMs opt to stay with proven thru-hole technology.
A system or product like this may be overly expensive to manufacture, but more than the budget, consider its proven longevity in the field or on the market, as well as its high quality and reliability. Even if newer, better, faster technologies become available for these systems, the likelihood of adopting them in these sectors is low to zero.
DfM, the Great Equalizer
DfM is the linchpin and equalizer for assuring accurate mixed assembly placement. Design and assembly methodologies typically used for SMT and inadvertently applied to thru-hole may incur problematic faults, during assembly and after the product ships to the OEM. It’s best to rely on proven experience and knowhow to avoid mixed assembly placement issues at the outset.
DfM issues at layout and design range from proper connector placement to bifurcating the layout so devices operate properly and aren’t subjected to various restrictions. Connectors should be placed close together, and close to the board’s edge to avoid interference with cables or wires exiting the board. Designers must keep in mind the pick-and-place machine tolerances and restrictions.
As for SMT and thru-hole components placed together, it’s important to allow sufficient space between the two types for physical separation. Normally, thru-hole components require more physical separation when they are placed next to each other. On a given PCB, more SMT components can be used compared to thru-hole, because of this nature. As a consequence, if the necessary care is not given in placing SMT and thru-hole components, it will require a second operation by hand instead of machine placement. Also, rework costs will increase as a result of ignoring this issue.
When placing thru-hole components on a double-sided PCB, design becomes more critical. Thru-hole components can’t be placed at the same location on the top and bottom side, as the leads from the top would interfere with those from the bottom, making a functional PCBA impossible.
Figure 3. SMT and thru-hole components are evenly distributed on this RF board to maintain signal integrity and SNR.
SMT and thru-hole components must be evenly distributed in terms of power, voltage, and current, especially in RF applications where signal integrity and signal-to-noise ratios (SNR) are critically important. SMT and thru-hole sections are isolated from each other and the rest of the board by special ground pour (Figure 3).
If there is a thru-hole portion on the analog section of the board that carries high current, the PCB designer must bifurcate that segment and distance it from the board’s digital section. Digital signals normally carry low amounts of current and voltage and are sensitive to higher current fluctuations and spikes. PCB designers also need to separate the planes carrying different types of loads from each other.
Manufacturing
Properly placing mixed thru-hole and SMT means dividing the components so that assembly is performed smoothly; the placement route for the pick-and-place machine is optimized. Accurate placement in these instances demands avoiding mechanical, height, and/or cable, chassis, and wire restrictions. As part of PCB design and layout, detailed assembly notes are critical to assure machine tolerances are accounted for in precise fine-pitch SMT placements. These tolerances are for pick-and-place machines, wave soldering equipment, AOI devices, and testers.
Sometimes at assembly stages, thru-hole placement is done manually, by hand. It also can be accomplished by machine. An axial placement machine suits large volumes of thru-hole components involved in a product, as well as when production quantities are huge. Most U.S. and European manufacturers don’t invest in these placement machines, as these assemblers typically use thru-hole about 10?15% of the time. SMT is used roughly 85?90%. In some Far East countries and other locations where price-sensitive consumer devices are built, axial thru-hole machines are used more often for placement.
DfM factors in limitations, largely due to ever-shrinking PCB real estate. More components with greater functionality are being stacked up on the board. That’s one reason the industry has begun adopting SMT connectors as I/O devices. Since these are so new, their assembly may require special tools and jigs. Also, if these connectors are fine pitch, extra care should be taken to provide test coverage on different board segments. When real estate is reduced drastically, spaces traditionally available in old designs disappear.
Mixed assembly affects manufacturing productivity, adding two additional steps: lead formation and protecting the holes while wave soldering occurs. Both incur extra costs and time. The first requires component formation equipment or extra human labor. The second process involves placing Kapton tape over the thru-hole devices, so that the solder from the wave equipment won’t creep into the hole barrels.
Rework
Cmponent leads are clipped after wave soldering (Figure 4). Subsequent rework issues can crop up if a lead is clipped too short, jeopardizing proper connection to the rest of the board. Here, a rework technician’s skill set and expertise is critical. It isn’t an automated aspect of the process.
Figure 4. Component leads are clipped after wave soldering. If clipped too short, connection to the board is jeopardized.
Also at rework, special care must be taken to use the correct solder pot for leaded components or lead-free ones. If the wrong one is used, there’s the possibility of contamination or lowered reliability, or both. This is especially true when a PCB is used with surface finishes of electroless nickel/immersion gold (ENIG) is used. Organic solderability preservatives (OSP) PCBs limit rework cycles to no more than two.
When lead-free rework is involved for thru-hole components, it’s possible to damage the board or its components. The probability of damage increases because higher temperatures are used to depopulate the component, thereby subjecting boards and components to additional stress.
Zulki Khan, founder and president, NexLogic Technologies, Inc., may be contacted at (408) 436-8150 ext 102; zk@nexlogic.com; www.nexlogic.com.
Intelligent GPS on the Autobahn
Last year, I prepared myself thoroughly for driving in Germany. I brought along Google directions for every stop on the way. Everything worked beautifully until I was caught in a one-and-a-half-hour stop on the Autobahn due to roadwork. This year, my rental car happened to have a built-in GPS system, but not one that I had ever experienced before. For those of you more technically advanced than me, please bear with me as I extol in this technological wonder. While I was driving toward my destination in accordance with the running GPS instructions, the GPS suddenly informed me that it was recalculating my route based on traffic congestions. It guided me along an alternative route to the airport without any of the devastating delays of the previous year. The GPS system in my rental car was receiving real-time traffic updates constantly, enabling it to navigate around congested areas.
This technology fascinates me not only because of its sophistication, but also because of the enormous benefits and value that it provides. It saves people time and aggravation. It may prevent drivers from being late for their appointments or, in my case a flight. And, who knows, it may even save gas since the car is not idling for hours in stop-and-go traffic. Somebody ought to calculate the potential productivity improvements in a country based on drivers using such an intelligent GPS system.
Then it dawned on me: This is the perfect analogy for a modern thermal profiler and real-time process management system. As opposed to the passive profilers that essentially simply draw a map of the current terrain (time/temperature profile), the data intelligent profiler acts as a GPS system by offering the following:
- current location and desired destination, in what profile you want to achieve;
- automatically shows how to reach said desired destination, just as a profiler shows exactly what zone temperatures and conveyor speeds are required;
- automatically and in real time provides alerts of any bottlenecks along the way, the “traffic problems” of a profile drifting out of spec, etc.
In other words, the profiler will first measure the time/temperature profile and how well it fits the available process window. Next, it will search all available oven recipes and select the most appropriate to put the profile in the sweet spot of the process window and production starts. During production, the automatic profiling system – in the background – will check that all profiles stay in spec. If something happens, the driver (excuse me, technician) will be automatically alerted in order to adjust the oven set points prior to continuing production.
Just like the GPS, a data intelligence profiler saves time (including the costly production downtime) and aggravation, and it will make sure that you reach your destination (product quality and production productivity). Like my intelligent GPS, once you try it you will wonder why you have not taken advantage of this technology sooner. Now, if the modern profiler could only help save gas as well.
Bjorn Dahle, president, may be contacted at KIC, 15950 Bernardo Center Drive, #E, San Diego, CA 92127; (858) 673.6050; Fax: (858) 673.0085; bjorn@kicmail.com; www.kicthermal.com.